We wish to understand the origins of structural specificity and molecular recognition in proteins. In the dimerization domain of the yeast transcription factor GCN4, apolar mutants of a conserved, buried Asn16 side chain allow the domain to form a mixture of dimeric and trimeric coiled coils. This induced structural polymorphism provides a unique opportunity to study the impact of core packing on structural uniqueness. The N16A mutant of GCN4 forms a mixture of dimers and trimers under physiological conditions, but addition of small amounts of benzene results in trimers. The crystal structure of this benzene-induced trimer reveals a benzene molecule bound in the cavity normally occupied by position 16's side chains in other GCN4 mutant trimers. To understand how a hydrophobic molecule can restore structural specificity to a coiled coil, we must examine the packing in both oligomeric states and solve the structure of the N16A dimer.